1. Field of the Invention
The present invention relates to a new device using capacitance to detect fingerprint characteristics which is simpler and less expensive to fabricate than previous devices used for this purpose.
2. Description of the Prior Art
Previous capacitive fingerprint sensors have used arrays of electrodes with individual electronic circuits associated with each set of electrodes in the array. An element in the array may have one electrode, with the finger representing a grounded capacitor plate as described in U.S. Pat. No. 6,016,355 or it may have two electrodes with the finger representing a floating capacitor plate as described in another prior art device. In both cases, the change in capacitance due to a fingerprint feature is sensed by an individual electronic circuit located adjacent to or underneath the electrodes. The output of these individual circuits are electronically multiplexed together to form an image of the fingerprint.
The electronic circuits must be built as an integrated circuit on a suitable substrate, most likely silicon. The size of the array is determined by the size of the human finger, and is ideally at least 1 inch high by 0.75 inch wide to accommodate an adult thumb. A semiconductor chip of this size is expensive to fabricate and difficult to produce with a high yield of good devices. The resolution desired is set by the dimensions of the ridges and valleys of the fingerprint and is in the neighborhood of 500 elements per inch, or 250,000 elements per square inch. A typical array will require 187,500 separate electronic circuits, one per element, for an array of that is slightly smaller than on square inch.
An alternate approach is to use an array of movable electrodes and to measure the capacitance changes as the electrodes are displaced by the ridges and valleys of the fingerprint as described in U.S. Pat. No. 4,353,056. The electrodes and their support structures can be fabricated using micromachining techniques. The substrate can be either silicon or a non-semiconductor material such as glass. The surface of the array must be protected by a thin layer of material which is thin enough and flexible enough to conform to the ridges and valleys of a fingerprint and yet hard enough to resist physical damage in normal use. This type of array can be operated by selectively driving individual rows of elements and sensing the signals picked up on individual columns, the electronic circuits being placed on the periphery of the array. A typical array will require 500 circuits for driving rows and 375 circuits for receiving column signals, for a total of 875 electronic circuits. This approach is more complex mechanically and simpler electronically.
A third approach is described in U.S. Pat. No. 6,160,904 and is similar to the present invention to the extent that it uses capacitance measurements with an array of electrodes with no underlying active circuits below the electrodes. It differs in that the array senses the finger surface in one axis only, using a linear array of drive electrodes which extend across the finger. Another difference is that one pickup electrode per array located at one end of the array is used, the array having only one conductive layer. This device does not produce a detailed image of the fingerprint, but a signal which is characteristic of the fingerprint. This characteristic signal changes due to spreading of the finger under pressure and to angular misalignment of the finger with the sensor providing unsatisfactory results.
The present invention provides an array of electrodes to sense the presence or absence of the ridges in a fingerprint through capacitive measurements. The electrode array does not require individual electronic circuits for each element and can be manufactured solely from metallic conductors and suitable insulators. Exciting signals are supplied from outside the array. This allows the array to be fabricated from metal and insulator layers on non-semiconductor substrates such as glass or plastic using normal deposition and patterning techniques such as, but not limited to, vacuum deposition, sputtering, or vapor decomposition followed by photolithography. The sensor could be made by printing, replication or electroplating means. Since there is no need for deformation, a top protective layer consisting of hard, rigid material can be used. Suitable materials include diamond, silicon dioxide, silicon nitride, aluminum oxide, or other hard material capable of being formed in thin films.
The electronic circuits required can be located around the periphery of the array. These circuits can be fabricated as semiconductor chips, fabricated separately, mounted on the substrate, and connected using thermal an-isotropic bonding (TAB), wire bonding, solder balls, or other suitable method, or, alternately, the electronic circuits can be fabricated on the substrate using thin film techniques similar to those used to fabricate liquid crystal displays.
The present invention can also be used to detect the presence of a different material, such as moisture and chemicals.